Chronic pain after breast cancer surgery (CPBCS) is the most common cause for long-term morbidity in women diagnosed with breast cancer, with an incidence as high as 72%. Similar to other chronic postoperative pain conditions, CPBCS could be resistant to treatment and last for years, burdening women with its physical, emotional and social consequences. Studies in humans showed that adopting the concept of preemptive analgesia, i.e., the use of pain relieving measures prior to surgery rather than after it, may eventuate in decreased postoperative pain levels. Moreover, the preemptive use of analgesic measures prior to surgery may prevent the progression of acute postoperative pain to chronicity, compared to the same measures taken after surgery. Diets enriched with soy protein decreased chronic post surgical pain in rodents, and possible in humans. This effect was most apparent in animals when the soy protein was supplemented preemptively, prior to surgery. Our primary objective in this randomized, double blind, phase II clinical trial is to test whether diet enriched with soy protein, given to women preemptively prior to breast cancer surgery, may prevent the development of CPBCS. Two hundred and twenty women, diagnosed with breast cancer and scheduled for breast cancer surgery will be recruited. The trial will comprise two phases: 1. A preoperative dietary manipulation phase that will last for 2 weeks, and 2. A postoperative follow up period, lasting for one year. In the first phase eligible candidates will undergo a basic dietary evaluation followed by stratification into 4 study groups, based on their pre-vs. postmenopausal status and the planned type of surgery, i.e., formal mastectomy vs. breast-conserving surgery. Women in each group will be randomized to receive either the soy or milk protein: 50% of the daily dietary protein will be converted to powdered experimental proteins, soy protein in the experimental group and milk protein in the control group. Women will be instructed to decrease their daily protein consumption by half for maintaining their diet isoenergetic and isoproteic, and to maintain the same fat and carbohydrate profile. During the second, post-surgical phase women will resume their normal diet. During a one year follow up participants will be evaluated for their pain levels, disability, depression, physical activity and utilization of analgesic medications. This will be done at baseline and 3, 6, and 12 months postoperatively. We assume that after a year the number of women developing CPBCS after consuming the soy-protein enriched diet will be 50% less that women consuming milk protein-enriched diet. If shown to be beneficial, this simple dietary manipulation may prevent one of the most severe sequelae of breast cancer.Chronic pain after breast cancer surgery is the most common cause for long-term morbidity in women diagnosed with breast cancer (15). Soy products are increasingly consumed by populations outside South East Asia due to their proven safety, feasibility and their association with the prevention of major ailments like coronary artery disease and cancer (60,68). In this trial we hope to show that consumption of a diet enriched with soy protein before breast cancer surgery may decrease the incidence of chronic pain after it. If indeed proven, this simple measure may prevent one of the most severe sequelae of breast cancer and improve the quality of life of millions of women worldwide.